1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits and particularly to silicon semiconductor integrated circuits having a radio frequency (RF) circuit processing an RF signal.
2. Description of the Background Art
In recent years, as mobile phones have widely been used and wireless LANs have practically been used, semiconductor integrated circuits that are used in such electronics have been noted and RF semiconductor devices have been noted in particular. To provide electronics with high performance, it is essential that an RF semiconductor device serving as a main component provide high performance have a small size and be produced inexpensively.
Conventionally, group III-V compound semiconductors having high electron mobility, such as GaAs, have been a main stream of materials subtrates used in an RF semiconductor device. Group III-V compound semiconductors, however, are much more expensive than silicon, typically used as a material for substrates of semiconductor devices and have been an obstacle to inexpensively producing RF semiconductor devices.
The recent rapid advance in silicon MOS transistor microfabrication technology has now allowed silicon MOS transistors to have a small gate length less than 0.2 xcexcm. Such silicon MOS transistors allow significantly improved transconductance Gm and have now achieved characteristics applicable as gigahertz RF semiconductor devices.
If a silicon MOS transistor can be used to fabricate an RF semiconductor device, a significant cost reduction can be achieved and it can also be expected that a baseband portion or any other similar logic circuit portion conventionally fabricated using silicon MOS process techniques is provided in the form of a single chip, and by System On Chip (SOP) a reduction in cost and that in area for mounting can also be achieved. Thus there is a demand for rapidly developing an RF semiconductor device using a silicon substrate and having more satisfactory characteristics.
As has been described above, a silicon RF semiconductor device has RF characteristics having attained a level sufficiently applicable as an RF semiconductor device. However, it has several disadvantages in SOPing with an RF switch circuit switching on/off an input and output of an RF signal (hereinafter an RF signal processing circuit other than the RF switch circuit will be referred to as a xe2x80x9cspecific RF circuitxe2x80x9d). In particular, if it is used in a radio frequency range of no less than the 5 GHz band, the RF switch circuit""s insertion loss is disadvantageously increased and SOPing can hardly be implemented.
In RF semiconductor devices that are used in mobile phones, wireless LANs and the like, as aforementioned, an RF switch circuit is a significantly important circuit. As shown in FIG. 18, typically a switch circuit 140 is configured by a transmission and reception switch circuit using a single pole double throw (SPDT) switch 140xe2x80x2. In reception, the switch receives an RF signal from an antenna 141 and transfers the signal to a reception portion""s low noise amplifier (an RF low noise amplification circuit) 150. In transmission, the switch receives an RF signal from a power amplifier 150xe2x80x2 and transmits the signal to antenna 141.
Fabricating SPDT switch 140xe2x80x2 using an MOS transistor 130 allows SOPing with another, specific RF circuit. A possible, simplest configuration of the SPDT switch is shown in FIG. 19 by way of example. Furthermore, FIG. 20 is an equivalent circuit diagram of the FIG. 19 SPDT switch with a transmission side ON and a reception side OFF. In this SPDT switch an insertion loss increases for the following reason:
In FIG. 20, Cd represents an MOS transistor""s source/drain junction capacitance. Through source/drain junction capacitance Cd a substrate resistance Rsi is connected to a circuit. Improving the MOS transistor""s ON characteristics entails reducing an ON resistance RON and a relatively large gate width of approximately 100 xcexcm to 400 xcexcm is accordingly used. As such, source/drain junction capacitance Cd would assume a relatively large value. Furthermore, the source/drain junction capacitance Cd impedance |z| is represented by 1/(2xcfx80xc3x97fxc3x97Cd) and it decreases for a gigahertz RF frequency range, since frequency f assumes a large value. Consequently, an RF signal flows to substrate resistance Rsi and an RF signal to be transmitted by a switch would have a loss in transmission, i.e., an insertion loss is introduced. As such, for an RF range (of no less than the 5 GHz band in particular) an insertion loss is significantly increased and a satisfactory switch circuit can hardly be fabricated.
An SOPed RF silicon semiconductor device needs to employ a substrate formed of silicon providing a small resistance (of approximately 10 mxcexa9 to 10 xcexa9) to prevent latch-up. As such, in an RF switch circuit, with a large source/drain junction capacitance Cd, as described above, the silicon substrate""s resistance Rsi contributes to a significant loss. As such, SOPing with another specific RF circuit has significantly been difficult.
The present invention contemplates a semiconductor integrated circuit that can also provide high performance and high reliability when SOPing is employed to provide an RF switch circuit on a silicon substrate to switch on/off an input and output of an RF signal.
In accordance with the present invention a semiconductor integrated circuit includes a silicon substrate and first and second MOS transistors formed on the silicon substrate. The silicon substrate has a first region and a second region identical in conductivity to the first region and having a lower dopant concentration than the first region. The second MOS transistor is formed on a main surface of the second region and configures an RF switch circuit switching on/off an input and output of an RF signal. The first MOS transistor is formed on a main surface of the first region and configures a specific RF circuit other than the RF switch circuit.
Thus the first MOS transistor that is biased can provide a depletion layer larger in width, as seen in the direction of the thickness of the silicon substrate, than the second MOS transistor that is biased can. As such, the second MOS transistor can significantly be smaller in source/drain junction capacitance than the first MOS transistor. As a result, the first MOS transistor can be used to configure the specific RF circuit, which requires small-current-leakage characteristics, and the second MOS transistor can be used to configure the RF switch circuit, which essentially requires reduced source/drain junction capacitance, to provide a semiconductor integrated circuit with the specific RF circuit and the RF switch circuit arranged together on a single silicon substrate and having satisfactory characteristics. Note that they can discretely be fabricated in a conventional MOS process fabrication process simply by using an additional photomask.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.